Generally, photoflash units may be classified as either a low voltage or a high voltage unit. The low voltage photoflash units usually employ a battery or a charged capacitor whereby a voltage in the range of about 1.5 to 15.0 volts is provided. The high voltage photoflash units ordinarily employ a piezoelectric element and provide a pulse voltage in the range of about 2000 to 3000 volts.
Also, it is a common practice to employ sequencing circuitry wherein a plurality of photolamps are sequentially activated by a voltage from either a low or high voltage source. Moreover, this sequential activation of the photolamps is usually controlled by a plurality of radiation switches connected on circuit with the photolamps and the voltage source.
The radiation switches may be either of the normally closed type or the normally open (N/O) variety, and the normally open switch appears to be the more common. As is known, the N/O type radiation switch is positionally located adjacent a photolamp and has a relatively high resistance prior to radiation impingement. However, activation of the nearby photolamp serves to provide the necessary radiation whereupon the radiation-responsive switch is converted from a high resistance or open circuit condition to a relatively low resistance substantially short circuit condition.
Ordinarily, the N/O radiation-responsive switches include a pair of terminals spaced about 0.04" to 0.08" apart and covered over with an insulating material which becomes electrically conductive upon exposure to radiant energy from a nearby lamp. Examples of such radiation-responsive switches and materials are provided in U.S. Pat. Nos. 3,969,065 and 3,951,582 wherein copper and silver salts are employed with a plurality of different combustible binders.
Silver salts are currently used in most of the radiation-responsive switches employed in sequentially operable multilamp photoflash arrays. However, the silver used in such switches must have a relatively high level of chemical purity which adds greatly to the already relatively high cost of silver salts. Moreover, the silver salts have a relatively limited range of activation as compared with a photolamp which has a temperature range which is both wider and less controllable than the activation range of the N/O switch. As a result, it has been found that a switch sensitive enough to be activated by a low temperature lamp will exhibit "burn off" or inactivation when energized by a high temperature lamp. On the other hand, a switch insensitive enough to resist "burn off" by a high temperature lamp will not be activated by a low temperature lamp whereupon an open circuit will result.
At present, it is a common practice to design a N/O radiation-responsive switch such that exposure to a low temperature lamp is sufficient to activate the switch. The problem of "burn off" due to an excess of radiant energy is compensated for by making the N/O switch larger than the activating aperture of a reflector as illustrated in FIG. 1. In this manner a switch activation gradient is provided between a completely vaporized area 5 and a completely inactivated area 7. However, a relatively large switch area tends to introduce problems of cracking during the switch drying process and, if the cracks are large enough, results in an undesired lamp failure. Moreover, a relatively large switch is undesirably expensive of materials.
Additionally, it is known that arc gaps may be used in a sequential photolamp array as evidenced by U.S. Pat. No. 3,742,298. However, the above-described structure is dependent upon the breakdown of the arc gap whereupon conduction across the arc gap is achieved.